This invention relates to windows for convertible tops, and more particularly, to a process for making a clear flexible plastic convertible window having a weatherable abrasion-resistant outer surface. The window can be folded and unfolded repeatedly without causing undesired self-adhesion of the contacting surfaces.
Convertible tops for automobiles, boats and other recreational vehicles are characterized by their familiar foldable top which is made of nylon or other rugged, flexible and waterproof material. The convertible top has an optically clear as window which is often made of a flexible plastic material that withstands folding. Clear plastic convertible windows are commonly made from optically transparent sheets of polyvinyl chloride (PVC) which are desirable because of their flexibility and weatherability. However, such flexible thermoplastic PVC sheets do not have a high level of resistance to abrasion damage. In fact, scratching of clear plastic convertible windows is the major problem confronted by end-users, as well as those involved at various levels in the business of manufacturing, transporting and installing foldable clear plastic windows in convertible tops.
In recent years the windows for convertible tops have been largely made from window glass to avoid the abrasion problems of clear plastic convertible windows. However, window glass is not an entirely problem-free alternative because window glass is substantially heavier than plastic; it is breakable; it is more expensive; and it is not foldable, and therefore additional space is taken up when the convertible top is folded. In certain models of cars a foldable clear plastic window is the only option because of space constraints.
Thus, there is a need for a transparent plastic convertible top window which is weatherable, foldable and scratch-resistant.
Briefly, one embodiment of this invention comprises a window for use in a vehicular convertible top comprising a thin, flexible, optically transparent plastic substrate sheet; and a thin, flexible, optically transparent, abrasion-resistant protective outer film layer adherently bonded as a permanent coating to at least one face of the plastic substrate sheet. The protective outer film layer comprises a transparent weatherable film-forming polymeric material reacted with a cross-linking agent to provide thermosetting properties including a useful level of scratch-resistance in the finished protective outer film layer. The protective film also includes a dispersed lubricant additive which provides a non-tacky self-lubricating exterior surface in dry film form that enables repeated folding and unfolding of the window without self-adhesion of the contacting surfaces.
In one form of the invention, the polymeric material contained in the protective outer coating predominantly comprises a polyurethane material. A minor amount of a polyvinyl chloride additive is included to promote adhesion of the finished coating to the plastic substrate window material which is typically made of PVC. A polyisocyanate cross-linking agent is reacted with the polyurethane material to produce the thermosetting properties in the finished film. A polyglycerol ester lubricant additive produces external surface lubricity in the finished dry film.
Thus, the invention provides an improved convertible window in which the coating adherently bonds as a permanent protective outer layer to the PVC core of the window. In its finished form the window has (1) abrasion resistance much greater than the PVC core, (2) weatherability, (3) transparency brought about by resistance to hazing of the protective coating material, and (4) release properties of the self-lubricating surfaces that allow repeated foldability of the window. The invention overcomes the disadvantages of window glass convertible top windows. The resulting flexible plastic convertible window also avoids abrasion defects from occurring during manufacturing, shipment to the installation site, installation of the window in the convertible top, and shipment of the finished product to the dealer. As a result of the invention, OEM rejections can be greatly reduced while providing considerable savings in the cost of labor and materials, not to mention the longer life of the convertible window itself during use.
These and other aspects of the invention will be more fully understood by referring to the following detailed description and the accompanying drawings.